The media, citing unnamed sources, said Thursday the suspect was filmed by surveillance cameras in the Brussels metro on Tuesday carrying a large bag alongside Khalid El Bakraoui, whom prosecutors have identified as a suicide bomber. RTBF said it is not clear whether the second suspect was killed in the attack.

El Bakraoui's brother was identified as one of two suicide bombers who targeted the Brussels airport the same day in attacks that killed at least 31 people and injured more than 200.

Prosecutors did not immediately respond to the reports.

On Wednesday, officials confirmed that the other suicide bomber at the airport was Belgian national Najim Laachraoui, the suspected ISIS bomb-maker behind the devices used in the Nov. 13, 2015, Paris attacks, and likely those used in Brussels, also.

The DNA of Laachraoui was verified as that of one of the attackers on Tuesday, after samples were taken from remains found at the blast site in Brussels airport, a European intelligence official and a French police official told the Associated Press.

CBS News correspondent Charlie D'Agata reported that, according to Belgian authorities, Laachraoui's DNA was also found on the explosives used in the gun and suicide attacks in Paris, which left 130 people dead. He was already Europe's most-wanted man when he walked into Brussels Airport on Tuesday morning.

The third man seen with Laachraoui and Ibrahim Al Bakraoui in the airport -- the man on the right in the photo below -- remains unidentified and, according to Belgian federal prosecutors, on the run.

In this image provided by the Belgian Federal Police in Brussels on March 22, 2016, three men who are suspected of taking part in the attacks at Belgium's Zaventem Airport and are being sought by police. The man at center has been the identified by the Federal Prosecutors office as Ibrahim El Bakraoui. The man on the left is believed to be Najim Laachraoui, and the man on the right remains unidentified.

AP

The timing of Tuesday's attacks may have been forced by the pressure Belgian police were putting on the terrorists, CBS News correspondent Holly Williams reported Wednesday.

An apartment in a quiet Brussels suburb may have been an important clue, but it came too late. It was rented under a false name, according to Belgian media reports, by a man who turned out to be Khalid El Bakraoui.

Inside the apartment, investigators said they found ammunition, an ISIS flag, and most importantly the fingerprints of Salah Abdeslam -- the alleged logistics man behind the Paris attacks.

That led to Abdeslam's capture three days later, but the apartment may have also linked him with the Bakraoui brothers.

The ammunition raised fears that another attack could be imminent. But less than a week later -- before they were found or stopped -- the two brothers carried out their deadly plan.

On a laptop discovered by police yesterday, Ibrahim El Bakraoui wrote that he didn't know what to do, and feared he would end up in a prison cell.

Connecting the dots in a case like this one is not easy work, and the Belgian intelligence agency has been criticized as understaffed and underfunded.